Operating Segments and Enterprise-Wide Information - Revenue by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 28, 2019	Sep. 29, 2018	Sep. 28, 2019	Sep. 29, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 140,604	$ 134,989	$ 410,835	$ 388,788
Probe Cards
Disaggregation of Revenue [Line Items]
Revenues	116,447	111,606	338,187	318,120
Systems
Disaggregation of Revenue [Line Items]
Revenues	24,157	23,383	72,648	70,668
United States
Disaggregation of Revenue [Line Items]
Revenues	33,665	40,127	106,297	105,668
United States | Probe Cards
Disaggregation of Revenue [Line Items]
Revenues	28,400	34,398	88,127	89,441
United States | Systems
Disaggregation of Revenue [Line Items]
Revenues	5,265	5,729	18,170	16,227
South Korea
Disaggregation of Revenue [Line Items]
Revenues	23,597	20,874	55,514	77,759
South Korea | Probe Cards
Disaggregation of Revenue [Line Items]
Revenues	22,779	19,437	50,596	71,863
South Korea | Systems
Disaggregation of Revenue [Line Items]
Revenues	818	1,437	4,918	5,896
China
Disaggregation of Revenue [Line Items]
Revenues	31,383	22,080	73,581	48,993
China | Probe Cards
Disaggregation of Revenue [Line Items]
Revenues	24,427	16,928	58,882	36,975
China | Systems
Disaggregation of Revenue [Line Items]
Revenues	6,956	5,152	14,699	12,018
Taiwan
Disaggregation of Revenue [Line Items]
Revenues	18,255	19,809	78,491	61,905
Taiwan | Probe Cards
Disaggregation of Revenue [Line Items]
Revenues	16,513	19,032	75,157	57,540
Taiwan | Systems
Disaggregation of Revenue [Line Items]
Revenues	1,742	777	3,334	4,365
Japan
Disaggregation of Revenue [Line Items]
Revenues	16,929	14,735	16,611	19,277
Japan | Probe Cards
Disaggregation of Revenue [Line Items]
Revenues	13,640	10,462	12,568	14,710
Japan | Systems
Disaggregation of Revenue [Line Items]
Revenues	3,289	4,273	4,043	4,567
Europe
Disaggregation of Revenue [Line Items]
Revenues	9,548	9,128	29,689	30,180
Europe | Probe Cards
Disaggregation of Revenue [Line Items]
Revenues	5,754	5,499	15,601	15,189
Europe | Systems
Disaggregation of Revenue [Line Items]
Revenues	3,794	3,629	14,088	14,991
Asia-Pacific
Disaggregation of Revenue [Line Items]
Revenues	5,665	7,230	43,454	41,905
Asia-Pacific | Probe Cards
Disaggregation of Revenue [Line Items]
Revenues	3,516	5,557	31,807	31,355
Asia-Pacific | Systems
Disaggregation of Revenue [Line Items]
Revenues	2,149	1,673	11,647	10,550
Rest of the world
Disaggregation of Revenue [Line Items]
Revenues	1,562	1,006	7,198	3,101
Rest of the world | Probe Cards
Disaggregation of Revenue [Line Items]
Revenues	1,418	293	5,449	1,047
Rest of the world | Systems
Disaggregation of Revenue [Line Items]
Revenues	144	713	1,749	2,054
Products transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues	138,885	133,442	405,885	384,289
Products transferred at a point in time | Probe Cards
Disaggregation of Revenue [Line Items]
Revenues	115,324	111,020	335,054	316,495
Products transferred at a point in time | Systems
Disaggregation of Revenue [Line Items]
Revenues	23,561	22,422	70,831	67,794
Services transferred over time
Disaggregation of Revenue [Line Items]
Revenues	1,719	1,547	4,950	4,499
Services transferred over time | Probe Cards
Disaggregation of Revenue [Line Items]
Revenues	1,123	586	3,133	1,625
Services transferred over time | Systems
Disaggregation of Revenue [Line Items]
Revenues	596	961	1,817	2,874
Foundry & Logic
Disaggregation of Revenue [Line Items]
Revenues	68,431	61,270	213,453	181,819
Foundry & Logic | Probe Cards
Disaggregation of Revenue [Line Items]
Revenues	68,431	61,270	213,453	181,819
Foundry & Logic | Systems
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
DRAM
Disaggregation of Revenue [Line Items]
Revenues	39,425	37,359	104,355	105,716
DRAM | Probe Cards
Disaggregation of Revenue [Line Items]
Revenues	39,425	37,359	104,355	105,716
DRAM | Systems
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Flash
Disaggregation of Revenue [Line Items]
Revenues	8,591	12,977	20,379	30,585
Flash | Probe Cards
Disaggregation of Revenue [Line Items]
Revenues	8,591	12,977	20,379	30,585
Flash | Systems
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Systems
Disaggregation of Revenue [Line Items]
Revenues	24,157	23,383	72,648	70,668
Systems | Probe Cards
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Systems | Systems
Disaggregation of Revenue [Line Items]
Revenues	$ 24,157	$ 23,383	$ 72,648	$ 70,668